Research and development is currently underway to produce underwater projectiles that travel at very high speeds using the phenomenon of “supercavitation.” A progenitor of such projectiles is the “Shkval,” which is a rocket-propelled torpedo that was developed by Russia and achieves a velocity of 250 knots (288 mph).
A supercavitating projectile's main features are a specially shaped nose and a streamlined, hydrodynamic, and aerodynamic body. The nose has a blunt leading surface that is referred to as a “cavitator.” When the projectile travels through water at speeds in excess of about one hundred miles per hour, the cavitator deflects water outward so fast that the water flow separates and detaches from the surface of the projectile. Since water pressure takes time to collapse the wall of the resulting cavity, the nose opens an extended bubble of water vapor.
Given sufficient speed, the cavity can extend to envelop the entire projectile except the nose. Once engulfed by the bubble, the drag experienced by the projectile is significantly reduced. As a consequence, a projectile moving in the cavity (“cavity-running”) can travel at far greater speeds for a given amount of thrust than a projectile that is moving in a conventional manner through water. A cavity-running projectile quite literally “flies” through the surrounding gas. In the absence of sustaining propulsion, the projectile loses supercavitation and eventually stalls due to drag. A secondary benefit of cavity running is that the motion stability of the projectile is enhanced.